Arthur J. Lustig, Ph.D. 12/15/08 1 Telomeres, the nucleoprotein structures present at the end of the chromosome, are necessary for terminal replication, chromosomal capping and genomic stability. The protein Mre11 is involved in the ATM (Tel1) telomere checkpoint pathway in part through the regulation of telomere addition. This process usually involves the addition of simple sequence tracts (G1-3T)n by the reverse transcriptase ribonucleoprotein telomerase We have identified a novel semi-dominant allele in the yeast model system that is capable of inducing a specific recombinational response in cells devoid of telomerase. This presence of this allele, mre11A470T, suggests the presence of an Mre11 motif involved in the to regulation of recombination in telomerase negative cells. In particular, this allele confers the ability to bypass senescence and cell death that occurs in most cells lacking telomerase through the induction of a novel class of recombinants. Three specific aims have been developed to understand this regulation of telomeric recombination. The first aim tests whether the conserved region downstream of Mre11A470T acts as a functional motif in the regulation of telomere recombination and tests the role of threonine phosphorylation in the mre11A470T phenotype. This will be conducted though an extensive analysis of site-specific mutations in conserved residues of the motif. The second aim tries to identify targets of this motif by several techniques including a) a direct physical assay, b) a candidate-based approach to define mutants that interact with the mre11 alleles, and c) the use of microarray analysis at the genomic level. Finally, the third aim will test the hypothesis that the regulation of one class of recombination, break-Induced replication (BIR), is the basis of regulation of the novel class of telomere recombinants in mre11A470T cells. This will be accomplished through a) an examination of subtelomeric BIR, b) the use of a BIR recombination model system, and c) and the effect of imperfect homology, such as occurs at telomeres, on BIR. Through these multiple aims, we hope to gain a mechanistic framework for the Mre11 recombinational motif. This study will provide insights into comparable processes in higher eukaryotes that have been liked to oncogenesis and senescence. .